A piezoactuator of the said kind is known from US 6 274 967 B1. The piezoactuator has a piezoelement constructed in multiple layers. In such a piezoelement a plurality of electrode layers and piezoelectric layers are stacked alternately one over the other. The piezoelectric layers consist of a piezo-ceramic material. The pretensioning device for the introduction of force into the respective volume of the individual piezoelectric layers consists of a hollow, cylindrical spring element, an actuator cover and an actuator bottom plate. The piezoelement together with both its end faces is pre-tensioned between the actuator cover and the actuator bottom plate by means of the spring element. A force is introduced into a total volume of each of the piezoelectric layers with the aid of the pretensioning device. A unidirectional compressive tension is applied to the piezoelectric layers along the stacking direction. Introduction of the force or compressive tension causes a switching of domains. The domains are preferably polarized transverse to the direction of force introduction or the stacking direction.
In order to introduce the force into the total volume of each of the piezoelectric layers, each piezoelectric layer has surface sections which face away from one another and are aligned parallel to the end faces of the piezoelement. These surface sections face toward either the actuator cover or the actuator bottom plate of the pretensioning device. The surface sections are the same size as the end faces of the piezoelement. The force is introduced into the total volume of the piezoelectric layer in each case via the total surface section of the piezoelectric layer.
The known piezoactuator is used for example to activate an injection valve in what is known as a common rail injection system. For this purpose it is necessary that both a defined displacement and a defined force can be transmitted along the stacking direction.
A dimension for the displaceability of the piezoelectric material in the direction of an applied electrical field strength is known as the piezoelectric loading constant d33. One possible way to obtain a relatively large displacement at a given value of d33 would be to increase the total height of the piezoelement. Alternatively a relatively large displacement can be obtained by introducing a force or a unidirectional compressive tension along the stacking direction of the piezoelement. For this purpose the statistically distributed ferro-electrical domains are switched by means of a so-called ferro-elastic process preferably transverse to the applied compressive tension or transverse to the stacking direction, for example in an unpolarized piezoelement. This gives rise to a permanent shortening of the piezoelement. This shortened piezoelement is electrically activated. Applying an electrical field parallel to the stacking direction causes domain switching with a preferred direction parallel to the applied electrical field. Significantly more domains are switched in comparison with the piezoelement that has no compressive pretensioning. As a result there is a greater displacement of the piezoelement in the stacking direction when compared to the piezoelement that has no compressive pretensioning.
For it to be possible to use this means to obtain greater displacement in a stacked piezoelement constructed in multiple monolithic layers, a force of over 100 N would be necessary in the case of, for example, a basic piezoelement surface area of 1×1 mm2. In the case of a basic surface area of 5×5 mm2 a force of around 2.5 kN would be needed. This can only be accomplished with the aid of a stiff spring with a corresponding loss of no-load displacement.
However, using compressive pretensioning to increase the displacement is not only a problem for piezoactuators on the macro-scale. In particular, using compressive pretensioning to increase displacement is unsuitable for producing a piezoactuator with a relatively large displacement and force translation on the micro-scale.